Page 40 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 19

(4) No requirement of biomass size processing to avoid extra energy
input for size reduction before the pretreatment.
(5) Operation in reasonable size and moderate cost reactors.
Pretreatment reactors should be of low cost by minimizing their
volumes, using appropriate materials, and keeping operating pressure
reasonable.
(6) Nonproduction of solid residues. Solid-waste formed during the
preparation of hydrolyzate should not present processing or disposal
challenges.
(7) Effectiveness at low moisture content. The utilization of low mois-
ture content raw material would reduce the energy consumption
during pretreatment.
(8) Obtaining high sugar concentration and fermentation compatibility.
The concentration of sugars from pretreatment process and enzyme
hydrolysis should be above 10% to ensure an adequate concentration
of fermentation product, for example, ethanol, and keep recovery
and other downstream costs manageable.
(9) Lignin recovery. Lignin and other compositions should be recovered
and make sure the downstream processing simply and for conversion
into valuable coproducts.
(10) Minimum heat and power requirements. Heat and power are the
main energy cost for pretreatment, and low energy consumption is
very important for pretreatment.
However, it should be noted that to meet all of the abovementioned
“strict criteria” is impossible, and compromise usually has to be made
because each pretreatment has its own disadvantages and advantages. To
combine the merits of different pretreatments, hybrid pretreatment can be
employed; however, such combination may increase the complicity of the
process and the cost. In the following sections, the operation modes,
mechanisms, and enzymatic digestibility of the pretreated substrates will
be discussed for different pretreatment technologies.

2.2 Physical pretreatment and mechanisms
Physical pretreatment aims to increase the surface accessibility of lignocel-
lulosic biomass to enzymes by reducing the size of the materials or
destructing the cell wall structure. There are many types of physical pre-
treatments, but they are generally classified into mechanical comminution